An image display panel used as a display generally has a construction in which a multitude of unit pixels, including light-emitting elements and transistor elements, are arranged. In such an image display panel as described above, the light emission of light-emitting elements is controlled by transistor elements in individual pixels and, thereby, an image is displayed.
Conventionally, a thin-film transistor (TFT), in which an inorganic semiconductor thin film made of a-Si (amorphous silicon) or p-Si (polysilicon) is used as a transistor element, has been used commonly in an image display panel. In general, however, an apparatus provided with large-scale vacuum processing equipment and high-temperature processing equipment is required in the manufacture of TFTs that use an inorganic semiconductor thin film. Therefore, the cost of manufacture has tended to be high. Particularly in recent years, an increase in the cost of manufacture has been remarkable since an increase in the screen size of a display is being promoted.
Hence, in recent years, a study has been made of applying a thin-film transistor (organic thin-film transistor) that uses an organic semiconductor thin film to an image display panel in place of the inorganic semiconductor thin film. Since the organic semiconductor thin film can be formed by a relatively low-temperature process, it is possible to simplify the manufacturing steps of the organic thin-film transistor and reduce the cost of manufacture thereof. In addition, since a low-temperature process allows a relatively heat-sensitive resin substrate to be also applicable, it is possible to reduce the weight of the image display panel as a whole. Furthermore, use of a resin substrate having flexibility makes a flexible panel available. Still furthermore, the organic semiconductor thin film can also be manufactured by means of wet coating, such as printing or coating, under the atmospheric pressure. In this case, the image display panel can be manufactured with extremely high productivity and at low costs.
Incidentally, such an image display panel as described above is generally manufactured by forming light-emitting elements after transistor elements are formed. In such a manufacturing method, an insulating layer called a bank is often formed in each pixel in the outer-circumferential part of a region in which a light-emitting element is formed. This bank is provided in order to prevent short-circuiting between electrodes that a light-emitting element has and divide off a light-emitting cell. This bank also functions as a partition for retaining a material solution within a predetermined region when a light-emitting layer made of a high-polymer material is formed using a wet coating method in cases where an organic EL element, for example, is formed as the light-emitting element. Since such a bank is made of a polyimide, a photoresist or the like and remains as a structure of the panel, a selection is made of a bank formed of a material superior in mechanical strength and heat resistance.
As an image display panel including a bank, there is known an active-matrix display unit, for example, in which a conduction control circuit containing a thin-film transistor and a thin-film light-emitting element are included in each pixel, a region in which an organic semiconductor film of a thin-film light-emitting element is to be formed is defined by an insulating film thicker than the organic semiconductor film, and the insulating film is constructed of a lower layer-side insulating film made of an inorganic material and an upper layer-side insulating film made of an organic material (see Patent Document 1).
In such a display unit, the insulating film functions as a bank. In addition, Patent Document 1 describes that the insulating film having the above-described construction can be formed into a thick film without causing damage on the thin-film light-emitting element.    Patent Document 1: International Publication No. 98/10862 pamphlet